In the structure of HIV TAR RNA, there is a base triple formed between an A-U base pair in the upper stem of TAR and a U residue in the bulge. However, we do not observe the imino proton consistent with this triple. In order to separate the contributions to the RNA structure from the Watson-Crick AU base pair in the triple from the Hoogsteen A-U base pair, we are examining the structure of two TAR variants. In the first variant, the A-U base pair is replaced with an A-C base pair, while in the second, the U in the A-U pair is deleted, and the Hoogsteen A-U pair is replaced by a C-G Hoogsteen pair. In addition, two synthetic base analogs are being incorporated into the TAR bulge. We have successfully incorporated dihydro-uridine by in vitro transcription into TAR, which provides an imino proton stable to exchange at the position of the putative base triple. Through a collaboration with Prof. Larry McLaughlin at BC, we will incorporate a C-nucleoside that has a non-exchangeable proton at the C2 position, which should provide several additional key NOEs to define the base triple geometry. For both of these variants, a system was developed where the TAR hairpin was divided into two separate strands. The wild type sequence and a deoxythymidine residue have been incorporated successfully, and this bipartite system forms the correct structure in the presence of argininamide.